The disclosure relates to improved and thin thermally and chemically strengthened glass articles and improved methods for strengthening glass substrates, and relates more particularly to thin glass articles simultaneously exhibiting a deep depth of compression and high surface compressive stress.
Thermally strengthened glass articles are strengthened by heating a glass substrate to an elevated temperature above the glass transition temperature of the glass, and cooling the surfaces of the substrate rapidly (“quenching”), while the inner regions of the substrate, insulated by the thickness and fairly low thermal conductivity of the glass, are cooled at a slower rate. This differential cooling produces a residual compressive stress (CS) in the surface regions of the thermally strengthened glass article, balanced by a residual tensile stress in the central region thereof.
Thermal strengthening is distinguished from chemical strengthening processes, in which surface compressive stresses are generated by changing the chemical composition of the glass in regions near the surface by a process such as ion diffusion. In some ion diffusion based processes, exterior portions of the resulting glass article may be strengthened by exchanging larger ions for smaller ions near the surface to impart a CS (also called negative tensile stress) on or near the surface.
Thermal strengthening and chemical strengthening processes are distinguished from mechanical glass strengthening processes in which exterior portions of the glass article are strengthened or arranged by combining two types of glass. In such processes, layers of glass compositions that have differing coefficients of thermal expansion are combined or laminated together while hot. For example, by sandwiching molten glass with a higher coefficient of thermal expansion (CTE) between layers of molten glass with a lower CTE, positive tension in the interior glass compresses the outer layers when the glasses cool, again forming CS on the surface to balance the positive tensile stress.
Strengthened glass articles have advantages relative to unstrengthened glass articles. The surface compressive stress of the strengthened glass articles provides greater resistance to fracture than unstrengthened glass. The increase in strength generally is proportional to the amount of surface compression. If a glass article possesses a sufficient level of strengthening, relative to its thickness, then when and if the sheet is broken, it will divide into small fragments with dull edges rather than into large or elongated fragments with sharp edges. Glass that breaks into sufficiently small fragments, or “dices,” as defined by various established standards, may be known as safety glass, and is often referred to as “fully tempered” glass, or sometimes simply “tempered” glass.
With at least thermally strengthened glass articles, because the degree of strengthening depends on the temperature difference between the surface and center of the glass sheet, thinner glasses require higher cooling rates to achieve a given stress. Also, thinner glass generally requires higher final values of surface CS and central tensile stresses or central tension (CT) to achieve dicing into small particles upon breaking. Accordingly, achieving desirable dicing behavior in thin glass articles (i.e., articles with a thickness of around 3 mm or less) using known thermal strengthening processes alone or in combination with other strengthening process has been exceedingly challenging if not impossible. Moreover, such thin glass articles often do not exhibit high surface compressive stresses, which prevent flaw or crack nucleation and/or growth. Accordingly, there is a need for thin glass articles exhibit deep depths of compression while also exhibiting high surface compressive stresses.